[defencegrid]

I get this equation from the calibration curve using UV/VIS spectrophotometry:

y = 0.0.7755x - 0.0780

So to calculate the concentration of my sample I have substituted in the absorbance of the standard into y which is 0.0225 and solved for x.

0.0325 = 0.7755x - 0.0780

0.0325 + 0.0780 / 0.7755 = x

I get a concentration of 0.142mg/L


I would also like to calculate concentration by using the formula: c = A/el

As I understand it, e (molar extinction coefficient) would be 0.7755 from the same equation:

0.0325 = 0.7755x - 0.0780

c = 0.0325/0.7755 x 1 = 0.045

I get a concentration of 0.045mg/L

Why do I get a different concentration value? There's something I'm missing.

[Borek]

As I understand it, e (molar extinction coefficient) would be 0.7755 from the same equation

This way you ignore -0.078, which is your background.

Mathematically your approach is like stating that

0.0325 = 0.7755x - 0.0780

and

0.0325 = 0.7755x

is the same equation.

It is not.